It sounds like you have a fair amount of experience so don’t be afraid to look beyond Higdon. Also, reading over this thread, I am most concerned about your long runs. With about 4 months to go, your long run is half the distance of what you want to run 4 months from now. As Zeke stated, it’s definitely not too soon to start building that distance up.
Given your goal, I would say that speedwork would be at the bottom of the list of important factors, maybe even falling off the list. The first thing you have to do is get comfortable with the long run. This is accomplished by both building up your weekly miles and building up your long run. The next thing you have to do, given your choice of marathon, is get comfortable on the hills. You can do this by mixing hills into your regular training. Later in your training, you can add hill repeats to further develop your strength on the hills and get in some low-impact speedwork.
21 weeks is not a whole lot of time but it’s definitely possible if you have already been running for a while and are coming in with a reasonable base. Also, without a goal of a real fast time, you have the freedom to take more time to develop your base – which is the key to the marathon – without being overly worried about speedwork.
I don’t think you need a “program” of any kind if you are the kind of person who will get in the needed training without having a piece of paper in front of you telling you what to do every day. If you do feel that you need or want one, though, don’t be afraid to look at the more “challenging” ones and definitely don’t be afraid to adjust whatever you choose as appropriate.